1. Field
This disclosure relates generally to mass flow controllers (MFCs), and more particularly to a system for and method of monitoring flow through MFCs in real time. As used herein the term “gas” includes the term “vapor(s)” should the two terms be considered different.
2. Overview
Mass flow controllers (MFCs) are devices for measuring and controlling the flow of gases. They are usually used to control the flow of gases during a semiconductor manufacturing process wherein the flow of gases into a semiconductor tool, such as a vacuum chamber, must be carefully controlled in order to produce high yield semiconductor products. MFCs are usually designed and calibrated to control specific types of gas at particular ranges of flow rates. The devices control the rate of flow based on a given setpoint, usually predetermined by the user or an external device such as the semiconductor tool itself. MFCs can be either analog or digital. They are typically designed to be used with pressure ranges of the inlet gases, with low pressure and high pressure MFCs being available. All MFCs have an inlet port, outlet port, a mass flow meter including a mass flow sensor and a proportional control valve. A system controller is used as a part of a feedback control system that provides a control signal to the control valve as a function of a comparison of the flow rate as determined by the setpoint with the measured flow rate as sensed by the mass flow sensor. The feedback control system thus operates the valve so that the measured flow is maintained at the flow rate as determined by the setpoint.
Such control systems assume that the MFC remains in calibration within certain tolerances. In order to test whether flow rates of a MFC are within the tolerances of calibration, the MFC is typically tested off line with such devices as mass flow verifiers. While off line testing is very accurate, there is always a problem that a MFC can become out of calibration during the running of a process (in real time), and not be detected until the process is completed. Often this can result in lower yields of semiconductor product, and even a complete failure resulting in the loss of the entire product yield. This can be expensive, and is clearly undesirable. What is needed is a device and method for continually testing the calibration settings of a MFC in real time while processes are being run.